1. Field of the Invention
The present invention relates to laser microscopes and to a method for controlling the same.
This application is based on Japanese Patent Application No. 2006-347892, the content of which is incorporated herein by reference.
2. Description of Related Art
A known laser microscope using a point scanning method employs a galvanometer mirror, an acousto-optic device, or an electro-optical element as a scanning unit.
Although the laser microscope using a galvanometer mirror can ensure a sufficient deflection angle of laser light, there is a problem in that the required video-rate cannot be realized because its scanning speed is slow.
There are known light scanning devices capable of scanning laser light at the required video rate using an acousto-optic device (for example, see Japanese Unexamined Patent Application, Publication No. HEI-8-327936), and using an electro-optical element (for example, see Japanese Unexamined Patent Application, Publication No. HEI-10-288798).
However, when using the acousto-optic device as a scanning unit, the deflection angle of the laser light is only about ±1°. When satisfying the numerical aperture and field number in a scanning optical system, the required deflection angle of the light is generally in an inversely proportional relationship to the beam diameter of the light. Accordingly, in order to ensure sufficient field number and numerical aperture using the acousto-optic device, the beam diameter of the laser light must be 10 times larger than when using a galvanometer mirror. Therefore, an extremely large acousto-optic device crystal is required to realize incident laser light having such a large beam diameter.
When the beam diameter of the laser light to be made incident on the acousto-optic device is large, the delay effect of an acoustic wave passing through the acousto-optic device crystal becomes large. Accordingly, a cylindrical lens effect occurs whereby the laser light emitted from the acousto-optic device crystal is wider in the uniaxial direction, resulting in the generation of an astigmatic difference when focusing the laser light with an objective lens, which causes low resolution.
In Japanese Unexamined Patent Application, Publication No. HEI-8-327936, a cylindrical lens is disposed at the rear side of the acousto-optic device to prevent the laser light from becoming wide due to the delay of the sound wave passing through the acousto-optic device. However, the width of the laser light emitted from the acousto-optic device varies according to the scanning speed of the laser light from the acousto-optic device; therefore the cylindrical lens must be changed every time the scanning speed is changed.
Even when the electro-optical element is used as the scanning unit, the deflection angle is only about 0.2°. Accordingly, Japanese Unexamined Patent Application, Publication No. HEI-10-288798 discloses widening the deflection angle by providing a plurality pairs of counter electrodes which are provided in the electro-optical element and which are arranged in series in the optical-axis direction.
However, because the thickness of the electro-optical element crystal disclosed in this publication is extremely small, i.e., about 200 μm, the beam diameter of the laser light that can be incident on it is about 150 μm at most. In order to ensure sufficient field number and numerical aperture using such an extremely small beam diameter, it is necessary to realize a deflection angle equal to 200° or more at the exit of the electro-optical element.